1. Field of the Invention
The present invention relates to flat display screens, and more particularly to screens, so-called cathodo-luminescent screens, whose anode supports phosphor elements separated one from another by insulating areas and which can be excited by electronic bombardment. The electronic bombardment requires the phosphors to be biased and can be generated by microtips, low extraction potential layers or a thermo-ionic source.
2. Discussion of the Related Art
To simplify the following description, only color screens including microtips will be described, but it should be remarked that the invention more generally relates to the various above-mentioned types of screens or analog.
FIG. 1 illustrates the structure of a color flat display screen including microtips.
Such microtip screens are mainly constituted by a cathode 1 including microtips 2 and by a gate 3 provided with holes 4 corresponding to the positions of microtips 2. Cathode 1 is disposed so as to face a cathodoluminescent anode 5, formed on a glass substrate 6 that constitutes the screen surface.
The operation and the detailed structure of such a microtip screen are described in U.S. Pat. No. 4,940,916 assigned to Commissariat a l'Energie Atomique.
Cathode 1 is disposed in columns and is constituted, onto a glass substrate 10, of cathode conductors arranged in meshes from a conductive layer. The microtips 2 are disposed onto a resistive layer 11 that is deposited onto the cathode conductors and are disposed inside meshes defined by the cathode conductors. FIG. 1 partially represents the inside of a mesh, without the cathode conductors. Cathode 1 is associated with the gate 3 which is arranged in rows. The intersection of a row of gate 3 with a column of cathode 1 defines a pixel.
The device uses the electric field generated between the cathode 1 and gate 3 so that the electrons are transferred from microtips 2. Thus, the electrons are attracted by phosphor elements 7 of anode 5 if suitably biased. In color screens, the anode 5 is provided with alternate phosphor strips 7r, 7g, 7b, each strip corresponding to a color (red, green, blue). The strips are separated one from the other by an insulating material 8. The phosphors 7 are deposited onto electrodes 9, which are constituted by corresponding strips of a transparent conductive layer such as indium and tin oxide (ITO). The groups of red, green, blue strips are alternatively biased with respect to cathode 1, so that the electrons extracted from the microtips 2 of one pixel of the cathode/gate are alternatively directed toward the facing phosphors 7 of each color.
The selection of the phosphor 7 (phosphor 7g in FIG. 1) that must be bombarded by the electrons generated by the microtips 2 of cathode 1 requires a selective control of the biasing of the phosphors 7 of anode 5 for each color.
FIG. 2 schematically illustrates a structure of the anode of a conventional color screen. FIG. 2 is a partial top view near the phosphor elements, representing an anode constructed according to known techniques. The anode strips 9, which are deposited onto substrate 6, are interconnected outside the useful surface of the screen, by color of phosphors 7. Strips 9 are to be connected to a control device (not shown). Two interconnection paths 12 and 13, of the anode electrodes 9g and 9b, respectively, are formed for two of the three colors of the phosphors (for example 7g and 7b). An insulating layer 14 (represented in dotted lines in FIG. 2) is deposited on the interconnection path 13. A third interconnection path 15 is connected, through conductors 16 deposited on the insulating layer 14, to the anode electrode strips 9r designed for the phosphors 7r of the third color.
Generally, the rows of gate 3 are sequentially biased to a voltage of approximately 80 volts whereas the phosphor strips (for example 7r and 7b in FIG. 1) which should be excited are biased to voltages of approximately 400 volts. The remaining strips (for example 7r and 7b in FIG. 1) are biased to a low or zero voltage. The columns of cathode 1 are connected to respective voltages ranging from a maximum emission voltage to a non-emission voltage (for example 0 and 30 volts, respectively). The brightness of a color component of all the pixels of a row is so determined.
The selection of the values of the biasing voltages depends upon the characteristics of phosphors 8 and microtips 10. Conventionally, below a voltage difference of 50 volts between the cathode and the gate, there is no electronic emission, and the maximum emission that is used corresponds to a voltage difference of 80 volts.
The conventional method for controlling such a color screen consists of forming several pictures per second, for example 50 to 60 pictures per second, which provides a duration of approximately 20 milliseconds to form each picture. This duration is referred to as frame duration.
As represented in FIG. 3, during a frame duration, three pictures, each corresponding to one color, are sequentially formed, i.e., strips R, G, B are sequentially connected, during the duration of the color subframe Tr, Tg, Tb to high voltages to be selectively active. Conventionally, the color subframes are successively generated without interruption or are separated by very short time intervals during which the rows/columns are inactive.
As represented in FIG. 4, during each color subframe, rows L1 . . . Li-1, Li, Li+1 . . . Ln are sequentially connected to a high voltage so that all the pixels of the corresponding row can be excited at a predetermined time. During the time a row is biased, the column conductors of the cathodes are set to voltages adapted to impart to the corresponding pixels the desired brightness.
A drawback of such a flat display screen occurs when, in at least one picture area, it is desired to display for a relatively long time, ranging from a few seconds to a few minutes, a uniform color corresponding to one of the three primary colors. For this purpose, the corresponding screen portion is biased during only one subframe out of three. Then, it can be remarked that color varies after a short period. This phenomenon is hereinafter referred to as color drift. In practice, this means that at least one of the phosphor strips adjacent to the biased strips starts to be luminescent.
The reason of this phenomenon is not clearly understood. It is thought that this phenomenon is due to the fact that the electrons accumulate in the insulating areas 8 between the phosphor strips and create conduction toward adjacent strips.
To avoid this phenomenon, the prior art has devised various techniques. One technique consists of separating by short time intervals the biasing of the anode strips between two successive color subframes and to apply a negative voltage pulse to the anode that has just been biased before positively biasing the next anode to be excited.
However, this method, which gives satisfactory results for the elimination of the color drift phenomenon, has the drawback of being relatively complex to implement because it complicates the provision of the anode supply voltages, which are high values voltages (some hundred volts). Moreover, the method is detrimental for the screen's brightness.
Also, in monocolor screens, voltage breakdowns often occur when the screen is operated for a long time.